.pi./4 shift quadrature phase-shift keying (QPSK) is known for narrowing bandwidth in digital demodulating systems. It does this without code interference by using a Nyquist filter. The linearly-modulated frequency is efficiently used, the signal phase locus is controlled by a differential identifier and a signal point-mapping circuit so as not to pass through the origin, and the envelope changes less than that of QPSK to be resistant to influence of non-linearity. Therefore, using a mid-class non-linear amplifier heightens electric power efficiency. Even when fading changes the received wave phase, .pi./4 shift QPSK blunts high-speed fading by applying delay detection.
FIG. 8 shows a .pi./4 shift QPSK signal spatial diagram. Phase-change quantity in QPSK is 0, .pi./2, .pi. and 3.pi./2. When the phase changes by .pi., the signal phase locus passes through the origin, which produces a large envelope change if its bandwidth is limited. On the other hand, in .pi./4 shift QPSK the signal phase locus does not pass through the origin, and so the envelope change is small, which makes this method curtail non-linear influence.
In FIG. 8, the .pi./4 shift QPSK signal point is alternately selected among the QPSK signal points of 0, +-.pi./2 and .pi. shown by white circles and the .pi./4 shift QPSK signal points of +-.pi./4 and +-3.pi./4 shown by black circles: the phase-shift is limited to +-.pi./4 and +-3.pi./4. That is, the minimum phase-shift magnitude is .pi./4.
FIG. 9 shows a conventional demodulation circuit for demodulating input signals modulated by .pi./4 shift QPSK.
In FIG. 9, the received signal is converted into IF signal at a previous stage not shown, and supplied to phase-detection circuit 100. Circuit 100 detects the IF signal instantaneous phase by a signal from local oscillator 101 and outputs the phase quantity as the phase output. This phase output is sent to phase difference circuit 102 as the first input and also sent to delay circuit 103 for delaying the cycle by one symbol. Circuit 102 receives the output of 103 as the second input, and so it analogously calculates the phase difference between the output of the present symbol and of the previous one to be output. This phase difference output is supplied to judging circuit 104, and its demodulated data is output.
Circuit 104 outputs data 0, 10, 01 and 11 when the phase differential output is around .pi./4, -.pi./4, 3.pi./4 and -3.pi./4, respectively, which can demodulate the input signal modulated by .pi./4 shift QPSK.